I recently saw some tutorials regarding transforming an old PC into a homemade firewall running PfSense. (Initially here, then here. They're good tutorials and worth the read.) Pfsense is a distribution of FreeBSD that is designed to act as a customized firewall and router.

I figured it looked like a cool project, but I'm not an expert on the hardware required for it, so I'd like to ask if anyone here has any experience with this. More concretely, I'm wondering how an old (or new) computer-turned-router would be able to broadcast a WAN signal the same way a router does. I'm assuming of course, that you would have to fit an antenna and a different kind of network card than what your would find in an old laptop, but to be honest, I don't know.

Also, I figured I could hook it up to a switch (which would require me to buy a switch). But if that is the case, I should also be able to hook it up to the router so that the network goes like this:MODEM ----> FIREWALL (PfSense) ----> ROUTER --(over WAN)--> PC

But either way, I'm just speculating. I was thinking of using the project purely as a learning experience, but I'm interested to know if anyone here has tried this kind of setup before and whether you think it could be practical/useful.

You don't need any specific hardware if you intend to turn your computer into a firewall. Just read up on it so you can configure it in a secure manner.

Obviously, as you already stated, if you want to make a computer a wifi router it would need a wifi network card and the relevant setup of routing tables etc. if you intend to do all that work by yourself. Otherwise the protocol DHCP is the way to go.

Another thing to notice is that if you intend to run a telephone cable straight into your computer-router you would also need a modem but from your description you'll use your existing router/modem for the modem function.

So for that to happen you will probably have to change your existing router configuration in order to convert it into a bridge. This is a functionality most routers provided by ISP's support.

I don't get why exactly you'd need a switch. Do you have a traffic bottleneck in your house you need to manage? (joking ofcourse). It would probably be cheaper to go and buy a few more interfaces(network cards) if you need to connect via cable.

Modem|Bridge->PfSense->router_PC->network

Keep in mind that unless you disable the routing and firewall capabilities of your existing modem/router you'll probably run into a bit of trouble configuring this setup (double routers & double firewalls) and I won't go into further detail unless you need it.

All in all it sounds like a cool project to give you an idea of things so go and do it!